Integrated circuits generally contain millions of individual electronic devices, each typically the size of a few micrometers, and interconnections thereamong. As a result, no "physical" tool is adequate for fabricating the complex patterns representing the devices and interconnections on a semiconductor substrate. Instead, microelectronic patterning is conventionally performed by radiation, e.g., light, x-rays or electron beams. The process of using an optical image and a photosensitive film to produce a pattern on a semiconductor substrate is known as "photolithography."
In photolithography, a film of a photoresist (photosensitive film) is first applied to the substrate. Radiation is then projected through a transparent plate, or "mask," on which has been created a desired pattern in an opaque material. The resulting image is focused on the photoresist-coated substrate, yielding areas of light and shadow on the substrate that correspond to the image on the mask plate. Following development and etching processes, where portions of the resist and substrate are removed, a pattern corresponding to that on the mask is etched in the substrate film.
During the infancy of microelectronics, the master image (pattern) was photographically reduced from a macroscopic original. The desired pattern was cut by hand into a colored plastic sheet and the image reduced to the desired size using room-sized reduction cameras. This method has been replaced by a pattern generator, an apparatus that accepts a computer-generated description of the device and analyzes it into individual picture frames. The pattern generator then scans a mask plate, "writing" the pattern on the mask plate using, for example, a high intensity electron beam, to expose each device or interconnect in the pattern.
Semiconductor devices may typically be made up of as many as fifty individual layers of silicon, polysilicon, silicon dioxide, metal and silicides. The pattern for each layer is contained on a mask called a reticle. Reticles are generally between one and ten times the actual size of the pattern they produce. The group of reticles that corresponds to all the layers of an integrated circuit (IC) is called a device series.
The pattern generation process, requiring point-to-point creation of a complex pattern on a semiconductor wafer, is generally a slow process. Usually only one of the multiple devices images to be printed on a semiconductor substrate is pattern-generated. The single device pattern, or reticle, is then replicated repeatedly using a step-and-repeat camera (commonly known as a stepper) to form the array of devices and interconnects that cover the semiconductor substrate. The reticle may also consist of an array of several patterns and is distinguished from a mask that contains patterns that can be transferred to an entire semiconductor wafer (or to another mask) in one exposure.
Not only is the production of a reticle a slow process, it is also an expensive process. Multiplying the cost of a single reticle by the number of reticles in a device series illustrates the cost of producing a single new IC. The reticles' cost is a significant component of the overall cost in the design and development of a new, or custom, semiconductor device. During the design and development of a semiconductor device, changes in the design, due to changes in the design specifications or unexpected results during testing, are often encountered. The changes in the design will necessitate new reticles to implement the changed design. This not only increases the cost of the development of the semiconductor device, but also delays the verification and production phases of the semiconductor device.
Accordingly, what is needed in the art is an improved method to manufacture an IC that overcomes the above-described limitations.